libsmack: clean up smack_accesses_add_modify()

smack_accesses_add_modify() was a hard function to follow because of
not well chosen variable names and unclear documentation.

This patch fixes some minor coding style errors, gives more logical
names to variables and provides more undestandable documentation for
this function.
Fixes for doxygen errors.

Doxygen errors that are regression from 3810596. Went in because
I didn't have doxygen installed.